Single-molecule localization microscopy is used to construct super-resolution images, but generally requires prior intense laser irradiation and in some cases additives, such as thiols, to induce on-off switching of fluorophores. These requirements limit the potential applications of this methodology. Here, we report a first-in-class spontaneously blinking fluorophore based on an intramolecular spirocyclization reaction. Optimization of the intramolecular nucleophile and rhodamine-based fluorophore (electrophile) provide a suitable lifetime for the fluorescent open form, and equilibrium between the open form and the non-fluorescent closed form. We show that this spontaneously blinking fluorophore is suitable for single-molecule localization microscopy imaging deep inside cells and for tracking the motion of structures in living cells. We further demonstrate the advantages of this fluorophore over existing methodologies by applying it to nuclear pore structures located far above the coverslip with a spinning-disk confocal microscope and for repetitive time-lapse super-resolution imaging of microtubules in live cells for up to 1 h.

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A pure quantum state can fully describe thermal equilibrium as long as one focuses on local observables. The thermodynamic entropy can also be recovered as the entanglement entropy of small subsystems. When the size of the subsystem increases, however, quantum correlations break the correspondence and mandate a correction to this simple volume law. The elucidation of the size dependence of the entanglement entropy is thus essentially important in linking quantum physics with thermodynamics. Here we derive an analytic formula of the entanglement entropy for a class of pure states called cTPQ states representing equilibrium. We numerically find that our formula applies universally to any sufficiently scrambled pure state representing thermal equilibrium, i.e., energy eigenstates of non-integrable models and states after quantum quenches. Our formula is exploited as diagnostics for chaotic systems; it can distinguish integrable models from non-integrable models and many-body localization phases from chaotic phases.

We investigated photophysical properties of a donor-acceptor-type conjugated polymer by means of ensemble and single-molecule spectroscopy as well as density functional theory (DFT) calculation. The polymer is based on an alkyne-linked 1,8-carbazole (Cz) and possesses a benzothiadiazole (BT) as an electron acceptor moiety. A comparison with model compounds demonstrated that the dimer structure is the spectroscopic unit of the polymer. Single-molecule two-color excitation fluorescence and fluorescence lifetime experiments showed that the polymer molecules displayed broad distributions of fluorescence intensity ratio and fluorescence lifetime. Together with the DFT calculation, we demonstrated that the twist angle between the Cz and BT moieties played a central role in deciding those spectroscopic properties of the polymer. This result was further supported by single-molecule fluorescence spectral measurement. The spectral measurement also suggested intersegment interactions within the single chains. Furthermore, single-molecule defocused and polarization fluorescence imaging suggested efficient exciton migration and trapping occurring within the single polymer chain. These experiments also revealed changes of the lowest energy site within the single polymer molecules.